1. Field of the Invention
This invention relates to a speech coding method and system for coding a speech signal with high quality by a comparatively small amount of calculations at a low bit rate, specifically, at about 8 kb/s or less.
2. Description of the Background Art
CELP speech coding is a known method of coding a speech signal with high efficiency at a bit rate of 8 kb/s or less. Such CELP method employs a linear predictive analyzer representing a short-term correlation of a speech signal, an adaptive codebook representing a long-term prediction of a speech signal, an excitation codebook representing an excitation signal, and a gain codebook representing gains of the adaptive codebook and excitation codebook.
It is already known that, with such CELP method, a better excitation codevector can be searched out to achieve an improved sound quality by using, when the excitation codebook is to be searched, simultaneous optimal gains as the gain of the adaptive codevector and the gain of the excitation codevector. Such speech coding method which uses, when the excitation codebook is to be searched, simultaneous optimal gains as the gain of the adaptive codevector and the gain of the excitation codevector is disclosed, for example, in Ira A. Gerson and Mark A. Jasiuk, "VECTOR SUM EXCITED LINEAR PREDICTION (VSELP) SPEECH CODING AT 8 KBPS", Proc. ICASSP, '90 S9.3, pp.461-464, 1990 (reference 1) and in M. Tomohiko and M. Johnson, "Pitch Orthogonal CELP Speech Coder", Lecture Thesis Collection I of '90 Autumnal Research Publication Meeting, Acoustical Society of Japan, pp.189-190, 1990 (reference 2).
Meanwhile, as a speech coding method which codes a speech signal with high efficiency at a bit rate of 8 kb/s or less, the CELP method which employs a linear predictive analyzer representing a short-term correlation of a speech signal, an adaptive codebook representing a long-term prediction of a speed signal, an excitation codebook representing an excitation signal and a gain codebook representing gains of the adaptive codebook and excitation codebook as described hereinabove is disclosed in Manfred R. Schroeder and Bishnu S. Atal, "CODE-EXCITED LINEAR PREDICTION (CELP): HIGH-QUALITY SPEECH AT VERY LOW BIT RATES", Proc. ICASSP, pp.937-940, 1985 (reference 3).
According to the conventional speech coding methods disclosed in reference 1 and reference 2, the excitation codebook has a specific algebraic structure, and consequently, simultaneous optimal gains of the adaptive codevector and excitation codevector can be calculated by a comparatively small amount of calculation. However, an excitation codebook which does not have such specific algebraic structure has a drawback that a great amount of calculation is required for the calculation of simultaneous optimal gains.
Meanwhile, according to the conventional speech coding method disclosed in reference 3, gains are not normalized, and consequently, the variety of gains is great, which makes the quantization characteristic of the speech coding system low.